1. Field of the Invention
The present invention relates to an agent for forming a surface-protecting layer on a coated paper. More particularly, the present invention relates to an agent for forming a surface-protecting layer on a coated paper, which comprises an aqueous solution of an alkali metal salt, ammonium salt or amine salt of a graft copolymer and an epoxy group-containing crosslinking agent incorporated in the aqueous solution.
2. Description of the Prior Art
With recent development of information instruments and labor-saving instruments, various information recording papers have been proposed and put into practical use. Among these recording papers, heat-sensitive recording papers are especially widely used in outputs of computers, printers of desk-type electronic calculators, recorders of various measuring devices, facsimiles, copying machines, automatic ticket vending machines and the like. The properties required for these recording papers have recently become severer. For example, in case of a heat-sensitive recording papers, not only heat-sensitive characteristics such as the antisticking property and the coloring property but also the oil-resistant and water-resistant preservability, that is, the property that disappearance or bleeding of the colored portion is not caused by oil, water or a chemical such as a plasticizer, are required. More specifically, a heat-sensitive recording paper is prepared by dispersing a lueco dye type colorant such as Crystal Violet, a color developer such as bisphenol A and a filler such as a wax into water, adding a binder composed of an organic polymeric substance to the dispersion to form a heat-sensitive coating liquid and coating this liquid on the surface of a starting high-quality paper to form a heat-sensitive layer, and coloration is caused in the heat-sensitive layer by a thermal head of a printer or the like. Accordingly, when a recording paper is used in the field where the colored portion of the recording paper falls in contact with a plasticizer or oil, the plasticizer or oil reacts with the colored colorant to cause disappearance of the formed color, as is well-known in the art. Moreover, it is pointed out that when a recording paper is used in the field where the recording paper falls in contact with water or an aqueous liquid, water permeates into the heat-sensitive layer to dissolve the binder into water, resulting in peeling of the colored portion.
As means for improving the oil-resistant and water-resistant preservability for overcoming the foregoing difficulties of heat-sensitive recording papers, there has been tried a method in which the amounts used of the binder and wax are increased at the step of forming the coating liquid within such a range that the coloring property is not degraded, and a heat-sensitive layer is formed by using this coating liquid. However, the improvement of the oil-resistant and water-resistant preservability according to this method is still insufficient and no practically satisfactory result can be obtained.
Further, the use of polyvinyl alcohol or carboxy-group-modified polyvinyl alcohols for a thermosensitive recording adhesive label is proposed in U.S. Pat. No. 4,370,370, and the use of a copolymer of acrylamide or methacrylamide with arcrylonitrile, methacrylonitril, or a lower alkyl or hydroxyl lower alkyl ester of acrylic or methacrylic acid as an agent for forming a surface-protecting layer on a coated paper is proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 57-167489 and 57-171795. However, even by these measures, the improvement of the oil-resistant and water-resistant preservability is still insufficient.